1. Field of the Invention
The present invention relates generally to a driver circuit for driving a piezoelectric actuator as used for actuating print wires or other forms of printing elements of an impact dot-matrix printer, and more particularly to such a driver circuit which ensures a reduced amount of a change of the operated position of a piezoelectric element due to variation in the amount of its residual strain o its non-operated position which varies with the temperature.
2. Discussion of the Prior Art
A piezoelectric actuator is known in the art of actuating print wires of an impact dot-matrix printer, or an ink-ejecting mechanism of an ink-jet printer. Such a piezoelectric actuator utilizes its piezoelectric property wherein the application of a voltage across a piezoelectric element causes mechanical deformation or strain thereof, which is amplified by a suitable mechanism, to obtain a necessary amount of actuating stroke.
It is recognized that the amount of strain or deformation, of a piezoelectric element when no voltage is applied thereto, i.e., the amount of its residual strain has a large degree of dependence on the temperature in the negative direction. On the other hand, the amount of deformation or displacement of the operating surface of the piezoelectric element caused by a given voltage is constant. Consequently, the non-operated and operated positions of the piezoelectric element before and after the application of the voltage are changed depending upon the ambient temperature, even though the operating stroke of the element is constant. This is a problem with the piezoelectric actuator.
Where a piezoelectric element is used for an impact or ink-jet dot-matrix printer, therefore, the impact pressure of the print wires or the ink-jet pressure tend to be changed with the temperature of the operating environment, even when the piezoelectric element is energized by a constant voltage. Thus, the printer suffers from inconsistent concentration or density of an ink material which forms a printed pattern of dots, or insufficient printing pressure which leads to printing failure of some dots.
To solve the above problem, it is proposed to attach to a piezoelectric element or an adjacent amplifying element, a suitable metal or other material whose residual strain has dependence on the temperature in the positive direction, so that a change in the amount of strain of such a material due to a variation in the temperature may compensate for a corresponding change in the amount of strain of the piezoelectric element.
The above solution requires the use of a complicated arrangement or difficult adjustment of the amplifying mechanism for the piezoelectric element.
Further, the temperature of the piezoelectric element is affected not only by the ambient temperature, but also by a heat generated due to resistance losses of the element itself or a driver circuit for the element, and a heat due to a mechanical friction of the print wires of a printer. In other words, the temperature of the piezoelectric element is changed largely depending upon an average duty cycle of the element as an actuator which drives the corresponding print wire o ink plunger to print dots at appropriate matrix positions to form printed characters or images. When the temperature variation resulting from this factor is considerable, the known mechanical compensation by using a suitable material as indicated above is not sufficient to completely eliminate the temperature dependency of strain of the piezoelectric element.